What Causes Frequent Stiffness In My Legs While Walking?

Ok hi I am 19 and i have no clue what is wrong with me.... for about 2 years now my legs have been having problems and it keeps getting worse as time goes on.... while i am walking both of my legs at the same time will go stiff but its only for about a minute.. when it happens at time i cant feel my legs its like they are there but i can not use them. over this past summer i would be walking or briskly walking and my legs would go stiff but i would fall my legs did not bend but i could not control them... I would just fall and it happened 3 times in 2 days... my legs act up almost every dsy or every other day someone please help me??